A thin layer forming apparatus using a chemical vapor deposition (abbreviated "CVD process") is known in the field of electronics as one of apparatuses for forming a thin layer on a substrate.
The CVD method, which is a composing method utilizing a chemical reaction, is appreciated to be a suitable method for forming a high quality thin film not easily damaged and having a good step-covering property as compared to a physical deposition process such as vacuum deposition and sputtering. Further, as compared to an MBE (molecular beam epitaxy) process requiring a high vacuum, the CVD process, not requiring such a high vacuum, can use an inexpensive apparatus and is therefore suitable for mass-production.
However, there still exist some problems to be solved to provide a higher quality thin film using the CVD process These problems are discussed below.
FIGS. 4 and 5 show gas flow patterns in a conventional thin film forming apparatus using the CVD process. In these drawings, reference numeral 1 refers to a reaction chamber, 2 to a nozzle for jetting material gas, 3 to a substrate, 4 to a heating susceptor for supporting and heating the substrate; 7 to a material gas flow from the nozzle 2.
FIG. 4 shows a pattern of the gas flow while the susceptor 4 is not heated and maintained at the ordinary room temperature (20.degree. C.). The apparatus is designed so that the material gas flow appropriately reaches the substrate unless the susceptor is heated as referred to above. However, when the susceptor 4 is heated (400.degree. C.) for promoting the chemical reaction, a turbulence occurs in the pattern of the material gas flow due to a heat convection. As a result, a desired supply is disabled, which causes deterioration in the quality of the deposited film such as disunimity of the film, generation of pin holes in the film caused by nuclei produced in the gas phase.
There is recently proposed a CVD apparatus of a reduced pressure system as an improvement to remove the drawback of the normal pressure reaction system referred to above. This relies upon its property of suppressing the heat convection to reduce the pressure of the reaction chamber and reduce the ascending force.
FIGS. 6 and 7 show forms of the material gas flow under a reduced pressure.
In FIG. 6 which shows the gas flow 7 while the susceptor 4 is not heated and maintained at the ordinary room temperature (20.degree. C.), circulating vortexes already occur. This is caused by the fact that since the speed of the material gas flow from the nozzle 2 is faster than that under the ordinary pressure and the bounce from the susceptor is rather large. In FIG. 7 which shows the gas flow 7 while the susceptor is heated (1000.degree. C.), there are produced such large vortexes that the gas having reached a vicinity of the substrate on the susceptor soar again due to an additional ascending force caused by the heat convection.
As referred to above, turbulences in the material flow are not completely removed by merely reducing the pressure of the interior of the reaction chamber. Therefore, mere use of a reduced-pressure reaction chamber in the CVD apparatus cannot suppress turbulences in the material gas flow, and cannot prevent deterioration of the film quality caused by gas flow turbulences.
Further, since pressure reduction causes the material gas flow to expand throughout the reaction chamber, the material gas pollutes the reaction vessel, and the reaction vessel, in turn, pollutes the material gas.
Under these circumstances, for purposes of forming a high quality thin film in a reduced-pressure reaction system, it is indispensable to provide a new method capable of controlling the gas flow so that the material gas does not contact the wall of the reaction chamber.